In the making of a multilayer integrated circuit substrate, there has evolved a, so called, MCP or metallized ceramic polyimide structure. This structure comprises a ceramic substrate on which is evaporated a first layer M1 of chrome-copper-chrome on which printed circuitry is formed on the copper, a layer of polyimide, and a second evaporated layer M2 of chrome-copper-chrome on which printed circuitry is formed on the copper. Via holes are formed in the polyimide, prior to the deposition of the M2 layer, which extend down to the copper circuitry of the M1 layer and which conductively connect the circuitry on the M1 and M2 layers.
The above basic structure has presented problems in that there is no solubility of copper in chrome and, as a result, in the vias the bond between the copper of the M1 layer and the chrome of the M2 layer is a mechanical bond or coupling. Also, the top chrome of layer M1 is removed chemically at the bottom of the vias leaving copper which is exposed to air prior to the deposition of the M2 layer which results in oxidation. As such, this bonding system which depends entirely on a mechanical coupling and is extremely sensitive to film oxidation and/or contaminants at the metal to metal interface did not provide the desired adhesion and also created undue electrical resistance in the vias.
Subsequently, it was proposed to modify the above process to improve the interconnection between the M1 and M2 layers by etching the top chrome layer of M1 and the bottom chrome layer of M2 and providing a solder connection between the copper layer of M1 and the copper layer of M2. This modification is described in the IBM Technical Disclosure Bulletin, Vol. 22, No. 10, March 1980, pages 4471-4473. The modified process proved to have several disadvantages. It was an expensive process. The use of a solder connection limited the other processes to be carried out on the substrate due to the low melting point of the solder. Also, trouble was encountered due to entrapped solder fluxes in the vias. In the processing of large quantities of substrates, a process was still required which was less time consuming and less costly and which was more reliable and would still provide the desired adhesion and low via resistance.